1. Field of the Invention
The present invention relates to image forming apparatuses, image forming methods, and process cartridges, of electrophotographic type, such as copiers, electrostatic printing, printers, facsimiles, and electrostatic recording.
2. Description of the Related Art
Electrophotographic images have been heretofore formed in a wide variety of manners; typically, a surface of a latent electrostatic image bearing member (hereinafter, sometimes referred to as “photoconductor”) is charged, then the charged surface of photoconductor is exposed to form an electrostatic latent image. Then the electrostatic latent image is developed by use of a toner, thereby to form a visible image on the photoconductor. The visible image is then is transferred directly or through an intermediate transferring member to a recording medium, then the transferred image is fixed by means of heat and/or pressure, thereby a recorded matter on which images being formed is produced. Residual toner on the photoconductor, after the transferring of images, is cleaned by conventional means such as blades, brushes, and rollers.
Full-color image-forming apparatuses on the basis of such electrophotographic processes are typically classified into two types. One is single or single-drum type, in which one photoconductor and four developing units for four colors of cyan, magenta, yellow and black are mounted in one image forming apparatus. In such single type, a four-color image is formed on a photoconductor or a recording medium. The single type may allow to share a charging, an exposing, a transferring, and a cleaning units, disposed around the photoconductor, thus making possible to downsize and lower the cost compared to tandem type.
Another is tandem or tandem-drum type, in which plural photoconductors are mounted on an image forming apparatus (see Japanese Patent Application Laid-Open (JP-A) No. 05-341617). In general, each one of charging, developing, transferring, and cleaning units are mounted per photoconductor to construct an image forming unit, and plural image forming units, typically four units, are disposed in an image forming apparatus. In the tandem type, visible images are successively transferred on a recording medium through forming one-color visible images by one image forming unit to form full-color images. The tandem type allows high-speed image formation since visible images of respective colors are formed in parallel. That is, the tandem type can shorten the image-processing period by one-fourth compared to the single type, thus leading to four-times high-speed printing. In addition, durability of the units of image forming units like photoconductors may be enhanced indeed. This is due to that four steps of charging, exposing, developing and transferring are carried out to form a full-color image as regards one photoconductor in the single type, whereas only one step of these steps is carried out as regards one photoconductor in the tandem type.
However, the tandem type suffers from larger and expensive systems due to plural image forming units.
For this countermeasure, the diameter of photoconductors is decreased, the respective units around photoconductors are downsized, and the image forming units are small-sized. Consequently, the image forming apparatuses are small-sized and thus material cost is correspondingly reduced, and the total cost can be reduced in a degree. However, the compacted and small-sized image forming apparatuses bring about new requirements for higher performance of the image forming units and significant stabilization thereof.
Recently, image forming apparatuses such as printers, copiers and facsimiles have been commercially demanded for energy conservation and higher speed. In order to achieve these properties, it is essential to improve heat efficiency of fixing units of image forming apparatuses.
In image forming apparatuses, unfixed toner images are typically formed on recording media such as recording sheets, printing paper, photosensitive paper and electrostatic recording paper in an indirect or direct way by image forming processes such as electrophotographic, electrostatic and magnetic recording processes. Contact-heating processes such as heat-roller, film-heating and electromagnetic induction-heating processes are employed generally for fixing the unfixed toner images.
The heat-roller fixing units are typically constructed from a fixing roller, capable of being controlled at a predetermined temperature by use of a heat source such as halogen lump disposed inside thereof, and a pressure roller being urged to press the fixing roller as a pair of rotating rollers. A recording medium is inserted and conveyed between the contacting portion, i.e. so-called nip portion, of the pair of rotating rollers, thereby unfixed toner images are fused and fixed by action of heat and pressure from the pressure roller.
The film-heating-fixing unit is, for example, disclosed in JP-A Nos. 63-313182 and 01-263679. In the film-heating-fixing unit, a heating member, fixed and supported by a support member, is contacted with a recording medium through a heat-resistant thin fixing film, then the fixing film is slid and moved against the heating member, thereby the heat is supplied from the heating member to the recording media through the fixing film.
The heating member is exemplified by a ceramic heater where an electric resistance layer is disposed on a ceramic substrate such as alumina and aluminum nitride having proper heat resistance, insulating property and thermal conductivity. The fixing unit, equipped with such a lower heat-capacity fixing film, may exhibit higher thermal conductivity than the heat-roller fixing units and shorten the warm-up period, and also allow quick-starting and energy-saving.
The fixing unit of the electromagnetic induction-heating processes is exemplified by electromagnetic induction-heating in which Joule heat is generated in a magnetic metal member through an eddy current by action of alternate magnetic filed to cause electromagnetic induction-heating of a heating member (see JP-A No 08-22206).
In the fixing unit of the electromagnetic induction-heating processes, a film with an elastomeric layer is disposed between a heating member and a recording medium in order to heat and melt visible images uniformly in a sufficient enclosing manner of the visible images. When the elastomeric layer is formed of silicone rubber, its lower thermal conductivity degrades thermal response, and thus the thermal difference is remarkably enlarged between the inside face of the film heated by the heating member and the out side of the film contacting with toner. As a result, surface temperature of belts may rapidly drop in cases of much deposited amount of toner, causing possibly so-called cold offset due to insufficient fixing ability.
In addition, the fixing unit of electrophotographic image forming apparatuses is typically demanded for releasing ability of toner with heating members (hereinafter, sometimes referred to as “offset resistance”). The offset resistance may be improved by the presence of release agent at toner surface. However, unusual toners or reuse of toners may reduce the release agent at the toner surface, possibly deteriorating the offset resistance.
In addition, as the electrophotographic technology has progressed, toner has been demanded for low temperature fixing ability and storage stability or blocking resistance. For example, such toners are proposed as a toner that contains a linear polyester resin with certain properties as molecular weight (see JP-A No. 2004-245854), a toner that contains a non-linear cross-linked polyester resin with rosins as an acid ingredient (JP-A No. 04-70765), and a toner of which the fixing ability is improved by use of rosin modified with maleic acid (JP-A No. 04-307557).
In addition, while high speed and energy saving of image forming apparatuses require toners with superior low temperature fixing ability, the high speed requires the offset resistance that is conflictive property with the low temperature fixing ability. In order to satisfy the both requirements, a toner is proposed in which a rosin monomer is added to a polyester (see JP-A No. 04-70765). A blending process is also proposed in which a lower molecular weight resin and a higher molecular weight resin are blended (JP-A No. 02-127657).
However, the blending process of a lower and a higher molecular weight resins (JP-A No. 02-127657) suffers from insufficient milling ability at the resin production process due to the higher molecular weight ingredient and/or at the production process of milled toner with the binder resin. On the other hand, in the process where only the higher molecular weight resin is used, the offset resistance and storage stability are insufficient, and also the productivity is lower due to resin fusion during milling process by virtue of higher milling property.
When the softening temperature of toner binder resin is lowered so as to enhance the low temperature fixing ability, the toner tends to agglomerate and deteriorate storage stability since the glass transition temperature is inevitably lowered.
While the rosins may effectively improve the lower-temperature fixing ability as described in JP-A Nos. 04-70765 and 04-307557, some types of rosins suffer from odor.
As such, image forming apparatuses, image forming methods and process cartridges are demanded currently that can utilize toners with superior low temperature fixing ability, higher offset resistance, and excellent storage stability and also with less generation of odor, can assure excellent fixing ability and temporal stability, and can provide high quality images for prolonged period.